Regular use of which of the following fertilizers increases the acidity of soil?

The soil becomes more acidic when ammonium sulphate is used often.
Sulphuric acid is produced during the hydrolysis of ammonium sulphate, which raises the acidity of the soil.

${\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\to 2{\mathrm{NH}}_3+{\mathrm{H}}_2{\mathrm{SO}}_4$

Slaked lime is applied to neutralise the acid in order to prevent acidity.